Add workspace file to `.gitignore` (#359)

Having a `workspace.code-workspace` file in the root of this workspace
will cause the Extension Development Host to automatically open the
specified folder.

I'm adding this to the gitignore instead of committing a workspace file directly
because I often want to specify a real project for testing, and that local path
obviously shouldn't be committed.

This workspace file will cause the Extension Development Host to open
the godot-tools folder as a workspace. Simply replace the path with a path
to a real project to open that instead.

.gitignore

@@ -7,3 +7,4 @@ test
 configurations/tmp.txt
 configurations/test.py
 .vscode-test
+workspace.code-workspace